During papermaking, after the paper has been fabricated, a coating may be added to certain papers to enhance surface characteristics such as color, reflectivity and printability. Starch solutions are commonly employed. Foam is a consistent problem in these starch solutions and coatings. These types of systems tend to be different than typical wet-end paper systems. The primary difference is the absence of fiber. Additionally, whitewater systems generally exhibit low viscosities, 100 cps or less, while these surface modifying systems can range from 500 up to 1000 cps or higher. The high viscosity of these systems is necessary to reduce the amount of penetration into the paper and allow the control of applied coating weights. Also, some of these types of coatings, like starch solutions, require elevated temperatures (140.degree. F. to 170.degree. F.) as a viscosity and gellation control causing entrained or dispersed air to be more easily retained and more difficult to control in these systems than in wet-end paper machine white water systems. The high speed coaters used to apply these solutions/coatings have a tendency to entrain or disperse air into these types of systems. The air retained in these systems leads to higher viscosities in turn resulting in lighter coating weights.
Additionally, air bubbles in the coating cause holes in the coating surface resulting in loss of sheet surface properties, sizing and ink bleed through. In many paper manufacturing processes, a sizing agent is added to the wet-end of the paper machine to obtain water and/or ink hold-out properties. Sizing agents added at this point are difficult to control and produce side effects, such as deposition in felts and dryer hoods, slipperiness of the finished sheet, and agglomeration of fillers. In recent years, there has been a shift to modify sheet properties at the size press or other dry-end coating equipment. Application of starch at the dry-end has been known for a long time. In recent years, synthetic sizing agents are being added to the starch solution for modification of surface properties. This enables papermakers to better control sheet surface properties, as well as costs. The drawback to this approach is that these sizing agents, generally copolymers of styrene-acrylic acid, styrene-maleic anhydride, and polyurethanes or modifications of these, are extremely foamy. This adds to the already hard to control entrained air/foam that is generated in these systems.
Conventional defoamer/anti-foam compositions include ethylene (bis) stearamide (EBS), fatty alcohol, fatty acid or hydrophobic silica. Problems result from the use of these products because they are particulate and will aggravate deposition. Additionally, some defoamer/anti-foam products may include a hydrocarbon oil carrier. Such products are known to negatively effect paper sheet properties, such as brightness and sizing.
These and other problems associated with the use of conventional defoamer/anti-foamers in the starch or coating process of a papermaking operation are resolved by the present invention.